Alodining

Disclaimer: The information presented below is believed to be
accurate. However, no warranty of any kind accompanies this information.

What is alodining?

Alodining is the chemical application of a protective chromate
conversion coating on aluminum.

Why would anyone want to alodine aluminum?

It provides good corrosion protection. It even protects when
scratched. Alodined 2024 aluminum withstands salt spray 150-600
hours before forming white corrosion. Untreated 2024 corrodes
in less than 24 hours.

It provides an excellent electrically conductive surface. This
helps to provide good electrical bonding in an airframe.

Paint sticks to it extremely well. In some cases, it can substitute
for primer.

Advantages of alodining compared to other coatings such as
primer or anodizing:

Adds no measurable weight.

Does not alter the dimensions of parts (does not make holes
smaller).

Requires essentially no cleanup after application. Encourages
treatment of all small parts as they are fabricated and installed.

Requires no electricity or skill to apply. (This is why I use
it !)

Disadvantages of alodining:

For the best possible job, requires two large tanks capable
of immersing each part. (Anodizing also requires similar large
tanks.)

Should not be applied if the tank temperature is below 70°F
(21°C).

The alodined surface is not as durable as anodizing or a good
paint.

Assembled parts cannot be alodined. (Neither can they be anodized.)

Disposal of spent tank contents can be a problem.

How much alodine solution is required to treat aluminum?

Aircraft Spruce, in their catalog, claims a gallon of alodine
solution will treat about 400 square feet of aluminum. The company
I bought mine from claimed 80 square feet. I estimate that my
RV-6A has between 1600 and 2600 square feet of aluminum surface.
(This is the total surface area of all aluminum parts in the whole
airplane.) Somewhere between 5 and 30 gallons would be needed.

How large are my tanks?

They measure 5 feet long by 1 foot wide and are filled to a depth
of about 2 1/2 feet. This translates to about 94 gallons, or enough
to do at least 3 aluminum airplanes like mine. I built the wooden
tank structures around custom-made liners of 0.100" black
polyethylene. These tanks are big enough to immerse the overwhelming
majority of the RV-6A parts, including many skins.

If I were building new tanks today, how large would I make
them?

I would make them 5 1/2 feet long by 1 foot wide and about 3 1/2
feet deep. I would fill them to the 3 foot level, or about 125
gallons. They would then accept about half of the parts the current
tanks cannot hold. Had I ordered them that size in the first place,
the price would probably have been the same.

How much did the tank liners cost, and where did I get them?

The liners cost $250 each plus tax (custom made to my dimensions).
I ordered them from a company called Multi Lining and Coating
in Bakersfield. Their phone numbers are 800-966-0650 and 805-589-0650.
I don't remember the address.

Isn't the large quantity of alodine expensive? Aircraft Spruce
gets $17.50 per gallon.

Aircraft Spruce charges a lot for water. I purchased a 10 lb can
of Iridite 14-2 powder which, when mixed with water, makes about
128 gallons. This 10 lb can cost $140 in January of 1994. This
translates to about $1.09 per gallon. The other tank holds a preconditioning
acid solution. Fifteen gallons of the concentrated acid called
Isoprep 184, enough for about 100 gallons of solution, cost $112.50.
Naturally, tax and freight are extra. I avoided freight charges
by picking up the acid and alodine in person at the company plant
in Los Angeles. At the time the company was Allied Kelite division
of Witco Chemical Corp. Since then, the division was sold to MacDermid,
Inc. I have not done business with them. I do not know anything
about the current product line, current prices, or even if the
Los Angeles plant is still in existence. MacDermid' s phone number
is 800-325-4158. The number is probably in Connecticut.

What is the exact process used to alodine a part?

Wipe the part with acetone to get all the ink, glue, and other
stuff off.

If the part is bright Alclad rub with very fine Scotchbrite
type pads to take the shine off. If the part is not clad (such
as an extruded angle). take off the oxide layer with aggressive
use of Scotchbrite type products such as polishing wheels or discs.
If the part is non-Alclad weathered 2024-T3 with a dark colored
surface (yes, Vans included a few parts like that in my kit),
much work is needed to clean it down to the bright aluminum. Alclad
parts that have been formed into bulkheads and ribs and have a
dull finish do not need to be rubbed with Scotchbrite. Freshly
machined parts also do not need any Scotchbrite.

Wash in Dawn dishwashing detergent and rinse thoroughly. (Do
not attempt to substitute a different detergent. All my attempts
at using a substitute have met with dismal failure.) After this
washing, the part must support an unbroken film of water. Areas
that the water film will not cover indicate grease. The part must
be washed again.

Immerse in the acid tank for 3 minutes.

Dip rinse in the acid rinse barrel.

Immerse in the alodine tank for 3 minutes. Move the part around
a little every 30 seconds or so.

Dip rinse in the alodine rinse barrel.

To avoid drip marks, blow dry the part. If drip marks do not
bother you, you can let the part air dry. Avoid rough handling
of the part for 12 to 24 hours. The fresh alodine film is easily
damaged, but will harden significantly over time. If you are careful,
the part can be riveted in immediately.

This process sounds horrible. Isn't painting a lot easier?

Well, with a few small parts, I can start at step 1 and do everything
up to step 9 in 15 to 20 minutes. I could never set up to paint
parts and clean up in that time.

URL: http://www.eaa1000.av.org/technicl/alodine/alodine.htm
Contents of The Leading Edge and these web pages are the viewpoints
of the authors. No claim is made and no liability is assumed,
expressed or implied as to the technical accuracy or safety of
the material presented. The viewpoints expressed are not necessarily
those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 10 July 1997